AIS is most commonly initiated by thrombotic vascular occlusion, followed by a complex series events including an inflammatory response that together leads to ischemic neuronal dysfunction. Notwithstanding intensive research, tissue-type plasminogen activator (tPA) is the only approved treatment for AIS. However, the brief therapeutic window and post-treatment bleeding complications have limited its clinical use. Therefore, there is a need to identify novel targets in the regulation of cerebral thrombosis and inflammation to dissociate physiological hemostasis and repair from pathological thrombus formation. In this proposal we focus on a recently discovered anti-inflammatory protein, developmental endothelial locus-1 (Del-1), which highly expressed in the brain that we hypothesize is also involved in the pathogenesis of thrombosis that leads to AIS. Our data demonstrate that Del- 1 has potent anti-thrombotic activity. We propose that Del-1 combines anti-thrombotic activity mediated through it phosphatidylserine (PS) binding domain with anti-inflammatory actions that have been attributed to integrin blockade. Down-regulation of Del-1 or transient depletion by increased uptake of Del-1 by PS-expressing cells at the onset of ischemic vascular injury may exacerbate thrombus formation and increase infarct size. Our preliminary data show for the first time that constitutive Del-1 deficiency is prothrombotic, increase infarct size and causes more extensive necrosis in a murine model of stroke. Intervention with a single dose of a fusion protein we created recombinant (r)-Del-1 with human IgG Fc fragment at nM concentrations rescues the phenotype of Del-/- mice. Our main hypothesis is that Del-1 is a novel brain derived hemostatic factor that plays a role in the pathogenesis and potentially the management of AIS. This will be tested in two interrelated aims: Aim 1. We will more fully characterize the impact of Del-1 deficiency on the development of thrombosis and AIS using two models of vascular injury, MRI imaging techniques to measure infarct size, and cortical blood flow. Aim 2. We will explore the potential of r-Del-1-Fc to prevent secondary expansion of pathological thrombosis while minimizing effects on hemostasis using 125I-fibrinogen and FITC-dextran extravasation. In follow-on studies, we will delineate the contribution of the PS and integrin binding domains to Del-1's role in the development of AIS. Successful completion of this project will identify Del-1 as a novel cerebrovascular endothelium derived homeostatic anti-thrombotic protein relevant to the pathogenesis of AIS. Although these studies are preliminary, the potential impact of Del-1 on pathological thrombosis and inflammation makes it an attractive new candidate to help prevent stroke expansion and recurrence.